The present invention relates to a hydrometallurgical process for treating zinc-laden dusts issuing from electric steel plan furnaces.
It is known that the raw materials used in electric steel plants are composed of scrap metal, such as galvanized metal sheets, compressed automobiles, etc . . . , which contain small proportions of heavy metals, such as Zn, Pb, Cu, Cr, etc . . . . At operational temperature of the electric furnaces (about 3000.degree. C.), these metals, which have a low melting point, instantly pass into vapour state (sublimation) and are thus contained in the fumes of the furnace. Dust is removed from these fumes by dry filtration before they are rejected into the atmosphere.
The chemical composition of the dust recovered by filtration is in direct relation with the composition of the scrap metal and special additives supplying the electric furnace. A systematic study of the composition of this dust, made on various samples in various steel plants, shows that they comprise about 21% by weight of Zn, 25% of Fe, as well as a small amount of lead and copper and other oxides. The means content of zinc, of the order of 20 to 21%, fluctuates little in time and in space.
The high temperatures (3000.degree. C.) prevailing in an electric furnace and the oxidizing conditions of the vapours determine the formation of zinc oxide ZnO and zinc ferrite ZnFe.sub.2 O.sub.4, compound of high temperature. For dust containing from 20 to 21% of Zn, 70 to 80% of the zinc is in the form of ZnO and 20 to 30% in the form of ZnFe.sub.2 O.sub.4. For a zinc content of 14 to 15%, this proportion is different, namely 50% in the form of ZnO and 50% of ZnFe.sub.2 O.sub.4.
This mainly zinc-laden dust is of no commercial value per se and is at present discarded in controlled dumps, this creating a serious problem of pollution by the heavy metals (Zn, Pb, Cr).